1. Field of the Invention
The present invention relates generally to radio frequency (RF) oscillators, and more particularly to RF oscillator harmonic frequency selecting circuitry.
2. Description of the Prior Art
Crystal oscillators typically are used for precision frequency control in oscillator circuits. The fundamental frequency of oscillation depends on the thickness of the crystal and its mode of oscillation. Crystal thickness is inversely related to the fundamental frequency so that the upper limit of operation for a crystal is determined by the minimum thickness possible without being so fragile that the crystal fractures. Due to the inherent physical limitations of crystal oscillators, a selected harmonic frequency of a fundamental oscillator frequency often is used for various high frequency applications. For example, to obtain a 920 MHz signal, the eighth harmonic frequency of a crystal oscillator rated at 115 MHz can be used.
Known harmonic frequency generating circuitry typically includes at least one frequency multiplier circuit and often several frequency multiplier stages in conjunction with an RF oscillator and a tuned circuit tuned to the fundamental frequency of the RF oscillator. Typically the tuned circuit provides a low level fundamental frequency output signal sufficient to drive a frequency multiplier. Inherent disadvantages resulting from such use of frequency multiplier circuits are the complexity and expense. Another disadvantage is that frequency multiplier circuits are difficult and time-consuming to adjust.